The yellowing can be caused by overwatering or by a need for iron in your alkaline soil. If your watering practices have not changed over the years, you may want to apply some Ironite around the soil beneath your hollies. Use in amounts as recommended on the label. It may take a few weeks to work, but the iron will help your shrubs take up the nitrogen and other nutrients in the soil, which will help the leaves turn a normal shade of green. Best wishes with your shrubs!